As a manufacturing method for effectively increasing the strength of the rod portion of a connecting rod, it is known to perform a hot forging step for hot forging the rod portion including a web consisting of a plate strip and two ribs projecting from either side edge of the web (so as to have an H-shaped cross section), a cold forging step using an open die assembly exposing both side edges of the web while enclosing the remaining part of the web and the ribs for cold forging the rod portion while allowing burr to protrude from either open side end of the open die assembly until the cross sectional area of the rod portion is reduced by 10 to 30%, and a step for removing the burr. (Patent Document 1)
It is also known to perform a cold forging of the rod portion without creating burr on the rod portion. The free ends of each rib are given with a relatively large thickness and are work hardened so that the moment of inertia of area of the rod portion may be increased, and the strength of the rod portion against buckling may be increased. See Patent Document 2. In this method, the die assembly includes an upper die assembly consisting of an upper rod die assembly, and a lower die assembly consisting of a lower rod die assembly. The die assembly further includes a pair of side die assemblies, a small end die assembly and a big end die assembly. Owing to a guiding action provided by a male tapered surface formed on the outer surfaces of the side die assemblies and the end die assemblies, and a female tapered surface formed on the upper rod die assembly, during the cold forging step, the side die assemblies and the end die assemblies are moved inward in synchronism with the downward movement of the upper rod die assembly so that the rod portion is pressed from four sides, or both vertically and laterally.
As a method of manufacturing a connecting rod having an excellent buckling strength and fatigue strength, it is known to form low carbon boron steel material into the shape of a connecting rod by hot forging, quench the forged connecting rod, cold forge the quenched connecting rod with a planar forging die assembly so that the ribs are plastically deformed by more than 0.2 mm in the height-wise direction and to be work hardened by being pressed in the vertical direction, and shot peen the cold forged connecting rod so that the fatigue strength may be improved. (Patent Document 3)